Resistance spot welding is used in a number of industries to join together two or more metal workpieces. The automotive industry, for example, often uses resistance spot welding to join together pre-fabricated metal workpieces during the manufacture of a vehicle door, hood, trunk lid, or lift gate, among other vehicle parts. Multiple individual resistance spot welds are typically formed along a peripheral region of the workpieces or at some other bonding region to ensure the vehicle part is structurally sound. While spot welding has typically been performed to join together certain similarly-composed metal workpieces—such as steel-to-steel and aluminum alloy-to-aluminum alloy—the desire to incorporate lighter weight materials into a vehicle platform has created interest in joining steel workpieces to aluminum or aluminum alloy (hereafter collectively “aluminum” for brevity) workpieces by resistance spot welding. Moreover, the ability to resistance spot weld workpiece stack-ups containing different workpiece combinations (e.g. aluminum alloy/aluminum alloy, steel/steel, and aluminum alloy/steel) with one piece of equipment would increase production flexibility and reduce manufacturing costs.
Resistance spot welding, in general, relies on the resistance to the flow of an electrical current through contacting metal workpieces and across their faying interface to generate heat. To carry out such a resistance welding process, a pair of opposed welding electrodes are typically clamped at aligned spots on opposite sides of the workpieces at a predetermined weld site. An electrical current is then passed through the workpieces from one welding electrode to the other. Resistance to the flow of this electrical current generates heat within the workpieces and at their faying interface. When the metal workpieces being welded are a steel workpiece and an aluminum workpiece, the heat generated at the faying interface initiates a molten weld pool in the aluminum workpiece. This molten aluminum weld pool wets the adjacent surface of the steel workpiece and, upon stoppage of the current flow, solidifies into an aluminum weld nugget that forms all or part of a weld joint between the two metal workpieces.
Resistance spot welding a steel workpiece to an aluminum workpiece presents certain challenges. For one, steel has a relatively high melting point and relatively high thermal and electrical resistivities, while aluminum has a relatively low melting point and relatively low thermal and electrical resistivities. As a result of these differences, aluminum melts more quickly and at a much lower temperature than steel during current flow. Aluminum also cools down more quickly than steel after current flow has ceased. Controlling heat balance between the two metals so that a molten weld pool can be rapidly initiated and solidified in the aluminum workpiece can therefore be challenging. It has been found, for example, that upon rapid cooling using standard industry practices, defects in the molten aluminum weld pool such as shrinkage voids, gas porosity, oxide residue, and micro-cracking are drawn toward and to gather at the faying interface. Additionally, prolonged heating during resistance spot welding—more specifically an elevated temperature in the steel workpiece due to its relatively higher resistance—is conducive to the growth of brittle Fe—Al intermetallic layers at the faying interface between the molten aluminum weld pool and the steel workpiece. These two conditions have been shown to reduce the peel strength and weaken the overall integrity of the ultimately-formed weld joint.